Zinc plating bath additive and alkaline non-cyanide zinc plating bath

ABSTRACT

Provided is a zinc plating bath additive enabling the rapid formation of a zinc coating having small variations in the thickness depending on the position on the surface of an object to be plated. The zinc plating bath additive contains a water soluble copolymer having, as the structural units, two amine compounds.

TECHNICAL FIELD

The present invention relates to a zinc plating bath additive to beadded to a zinc plating bath used in electroplating and an alkalinenon-cyanide zinc plating bath.

BACKGROUND ART

Zinc plating has been used for metal parts in order to prevent them frombeing corroded by rust. Electroplating is used for zinc plating of metalparts. Electroplating is a method of supplying a current to a metalwhich is an object of zinc plating (which metal will hereinafter becalled “object”) while dipping the object in a zinc plating bath. When acurrent is supplied, zinc dissolved in a zinc plating bath deposits onvarious positions on the surface of the object. The zinc thus depositedthen covers therewith the surface of the object and thus a zinc coatingis formed thereon.

In electroplating, variations in the deposition amount of zinc dependingon the position on the surface of the object have become a problem. Whenthe deposition amount of zinc differs, depending on the position on thesurface of the object, a zinc coating different in thickness is formedon the surface of the object. Due to the variations in the thickness ofthe zinc coating, the zinc coating is likely to be inferior in brightappearance and the like. In order to reduce the variations in thedeposition amount of zinc, an additive serving to suppress deposition ofzinc is added to a zinc plating bath (for example, Patent Document 1).Due to the action of this additive, the deposition amounts of zinc aremade substantially equal to each other at various positions on thesurface of the object, making it possible to form a zinc coating whosethickness does not vary so much among the positions on the surface ofthe subject.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP-A-51-135837

The above-described additive however suppresses deposition of zinc notonly at a position where the deposition amount of zinc becomes excessivebut also at a position where the deposition amount of zinc is originallysmall. This leads to the problem of a reduction in a formation speed ofa zinc coating.

With the foregoing in view, an object of the present invention is toprovide a zinc plating bath additive and an alkaline non-cyanide zincplating bath enabling the rapid formation of a zinc coating having smallvariations in thickness depending on the position on the surface of anobject.

SUMMARY OF THE INVENTION

The present invention has been completed with a view to satisfying theabove-described object. Described specifically, the zinc plating bathadditives and alkaline non-cyanide zinc plating baths described beloware provided by the present invention.

[1] A zinc plating bath additive containing a water soluble copolymerhaving structural units (a) each represented by the following formula(1) and structural units (b) each represented by the following formula(2):

[2] The zinc plating bath additive as described above in [1], whereinthe structural units (a) and the structural units (b) are contained inthe water soluble copolymer so that a percentage of the mass of thestructural units (a) relative to the sum of the mass of the structuralunits (a) and the mass of the structural units (b) is from 10 to 60%.

[3] The zinc plating bath additive as described above in [1] or [2],wherein in the water soluble copolymer, the structural units (a), thestructural units (b), or the structural unit (a) and the structural unit(b) have been linked to each other via a linking group represented bythe following formula (3) and/or the following formula (4):

(in the formula (3), a stands for an integer from 1 to 5, b stands foran integer from 1 to 5, d stands for an integer from 1 to 5, e standsfor an integer from 1 to 5, and m stands for an integer from 0 to 5,while in the formula (4), f stands for an integer from 1 to 5, g standsfor an integer from 1 to 5, h stands for an integer from 1 to 5, istands for an integer from 1 to 5, and n stands for an integer from 0 to5).

[4] An alkaline non-cyanide zinc plating bath containing the zincplating bath additive as described above in any of [1] to [3], zincions, and hydroxide ions.

[5] The alkaline non-cyanide zinc plating bath as described above in[4], wherein the water soluble copolymer is contained in an amount offrom 0.1 to 50 g/L.

[6] The alkaline non-cyanide zinc plating bath as described above in [4]or [5], containing N-benzylpyrimidium-3-carbonate.

[7] The alkaline non-cyanide zinc plating bath as described above in anyof [4] to [6], containing a bisulfite adduct obtained by adding abisulfite to the aldehyde group of an aromatic aldehyde compoundrepresented by the following formula (5):

(in the formula (5), R¹, R², R³ each represents —H, —OH, or —OCH₃ and R¹to R³ may be the same or different).

[8] The alkaline non-cyanide zinc plating bath as described above in[7], wherein the aromatic aldehyde compound is at least one compoundselected from the group consisting of 4-methoxybenzaldehyde,4-hydroxy-3-methoxybenzaldehyde, 3,4-dimethoxybenzaldehyde,3,4-methylenedioxybenzaldehyde, 2-hydroxybenzaldehyde, and4-hydroxybenzaldehyde.

[9] The alkaline non-cyanide zinc plating bath as described above in [7]or [8], wherein the bisulfite adduct is contained in an amount of from0.005 to 1.0 g/L.

[10] The alkaline non-cyanide zinc plating bath as described above inany of [4] to [9], wherein the zinc ions are contained in an amount offrom 5 to 25 g/L.

[11] The alkaline non-cyanide zinc plating bath as described above inany of [4] to [10], containing at least one compound selected from thegroup consisting of silicon compounds, sodiumethylenediaminetetraacetate, and trisodiumhydroxyethylenediaminetriacetate.

[12] The alkaline non-cyanide zinc plating bath as described above inany of [4] to [11], containing a heterocyclic compound represented bythe following formula (6) and/or a derivative of the heterocycliccompound:

(in the formula (6), p stands for an integer from 1 to 3 and R⁴ and R⁵may be the same or different and each represents —H, —CH₂COOH,—CH₂CH₂OH, or —CH₂CH(OH)CH₂Cl).

The zinc plating bath additive of the present invention enables therapid formation of a zinc coating having small variations in thicknessdepending on the position on the surface of the object. The alkalinenon-cyanide zinc plating bath of the present invention enables the rapidformation of a zinc coating having small variations in thicknessdepending on the position on the surface of the object.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a cathode sheet after a hull cell platingtest.

FIG. 2 is a graph showing the thicknesses, at various positions of acathode sheet, of each of the zinc coatings of Examples 1 to 4 andComparative Examples 1 and 2.

FIG. 3 is a graph showing the thicknesses, at various positions of acathode sheet, of the zinc coating of Comparative Example 3.

MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will next be described referring todrawings. The present invention is not limited to or by theseembodiments and it can be changed, modified, or improved withoutdeparting from the scope of the present invention.

1. Zinc Plating Bath Additive:

The zinc plating bath additive of the present invention contains a watersoluble copolymer having structural units (a) represented by thefollowing formula (1) and structural units (b) represented by thefollowing formula (2):

The water soluble copolymer has a weight average molecular weight (Mw)of preferably from 2000 to 50000 from the standpoint of its functionalcharacteristics. The term “functional characteristics” as used hereinembraces a characteristic of improving the appearance of a zinc coating(preventing the burnt deposits of the zinc coating and preventing unevenbright appearance), a characteristic of forming a zinc coating rapidly(characteristic of increasing a deposition amount of zinc per hour), anda characteristic of decreasing the variations in coating thicknessdepending on the position on the surface of the object (characteristicof forming a zinc coating with more uniform thickness). From thestandpoint of enhancing the above-described functional characteristics,the water soluble copolymer has a weight average molecular weight ofmore preferably from 2000 to 30000. The weight average molecular weightcan be determined using a calibration curve, that is, a third-orderapproximation curve, with polyethylene oxide (PEO) as a standard sample.

In addition, the zinc plating bath additive of the present inventioncontaining a water soluble copolymer having structural units (a) andstructural units (b) is effective for suppressing excessive depositionof zinc. Due to this effect, it is possible to suppress an excess amountof zinc from depositing only at limited positions on the surface of theobject and as a result, to suppress variations in the thickness of thezinc coating depending on the position on the surface of the object.

Electroplating may cause variations in current density on the surface ofthe object depending on the positions. When they occur, the depositionamount of zinc per hour increases at a high current density position onthe surface of the object, while the deposition amount of zinc per hourdecreases at a low current density position on the surface of theobject. Variations in the current density on the surface of the objectdepending on the position lead to variations in the thickness of thezinc coating depending on the position on the surface of the object.

The zinc plating bath additive of the present invention containing awater soluble copolymer having structural units (a) and structural units(b) can strongly suppress the deposition of zinc at a high currentdensity position. On the other hand, the zinc plating bath additive ofthe present invention containing a water soluble copolymer havingstructural units (a) and structural units (b) has a weak effect onsuppressing deposition of zinc at a low current density position. Evenaddition of the zinc plating bath additive of the present invention to azinc plating bath does not easily cause a decrease in the depositionamount of zinc per hour at a low current density position. Addition ofthe zinc plating bath additive of the present invention to a zincplating bath can therefore decrease the deposition amount of zinc at ahigh current density position, while keeping the deposition amount ofzinc at a low current density position. The deposition amount of zinc ata high current density position approaches the deposition amount of zincat a low current density position, resulting in small variations in thethickness of the zinc coating depending on the position on the surfaceof the object. In addition, the addition does not excessively suppressdeposition of zinc at a low current density position, so that a zinccoating can be formed rapidly.

The zinc plating bath additive of the present invention tends toincrease the deposition amount of zinc per hour with an increase in thepercentage of the mass of the structural units (a) contained in thewater soluble copolymer relative to the sum of the mass of thestructural units (a) and the mass of the structural units (b) containedin the water soluble copolymer (which percentage will hereinafter becalled “structural units (a)•(b) percentage”). Accordingly, thedeposition amount of zinc per hour can be increased or decreased byadjusting the structural units (a)•(b) percentage in the water solublecopolymer, without changing the zinc concentration of the bath orchanging the temperature of the bath. This means that the formationspeed of a zinc coating can be adjusted by adding the zinc plating bathadditive of the present invention to a zinc plating bath, withoutchanging the zinc ion concentration in the bath or changing thetemperature of the bath. As a result, there does not occur a problem,that is, an excessive increase in the thickness of a zinc coating atsome positions on the surface of the object, which will otherwise occurwhen the zinc concentration in the bath is increased or the bathtemperature is raised. In short, addition of the zinc plating bathadditive of the present invention to a zinc plating bath makes itpossible to overcome the problem which has occurred in the conventionalprocess upon adjusting the formation speed of a zinc coating.

In the zinc plating bath additive of the present invention, thestructural units (a)•(b) percentage in the water soluble copolymer ispreferably from 10 to 60%. When the structural units (a)•(b) percentageis from 10 to 60%, the formation speed of a zinc coating can be adjustedby increasing or decreasing the structural units (a)•(b) percentage inany mode of electroplating (barrel plating, zinc plating whilesuspending an object from a jig, or the like).

In addition, when the structural units (a)•(b) percentage in the watersoluble copolymer is from 10 to 60%, a difference in the depositionamount of zinc per hour is likely to decrease between a high currentdensity position and a low current density position with a decrease inthe structural units (a)•(b) percentage. In order to form a zinc coatingwith more even thickness, it is therefore recommended to incorporate awater soluble copolymer having a small structural units (a)•(b)percentage in the zinc plating bath additive.

The zinc plating bath additive of the present invention can exhibit theabove-described effect even when the structural units (a), thestructural units (b), or the structural unit (a) and the structural unit(b) of the water soluble copolymer are linked to each other via alinking group represented by the following formula (3) and/or thefollowing formula (4):

(in the formula (3), a stands for an integer from 1 to 5, b stands foran integer from 1 to 5, d stands for an integer from 1 to 5, e standsfor an integer from 1 to 5, and m stands for an integer from 0 to 5,while in the formula (4), f stands for an integer from 1 to 5, g standsfor an integer from 1 to 5, h stands for an integer from 1 to 5, istands for an integer from 1 to 5, and n stands for an integer from 0 to5).

The linking group represented by the formula (3) isbis(2-chloromethyl)ether, bis(2-chloroethyl)ether, orbis(2-chloropropyl)ether and it can be inserted in the water solublecopolymer by the linking in the form of a self polymerization productthereof in an aqueous solution reaction. The linking group representedby the formula (4) is epichlorohydrin, 1,3-dichloro-2-propanol, or1,4-dichloro-2-butanol and it can be inserted in the water solublepolymer by the linking in the form of a self polymerization productthereof in an aqueous solution reaction. For example, an urea derivativehaving the structural units (a) and an urea derivative having thestructural units (b) are mixed and dissolved in a water used as asolvent at a desired molar ratio and then, a necessary amount ofepichlorohydrin is added in small portions under stirring while keepingthe liquid temperature at 60° C. or less. The reaction mixture is thenrefluxed for 2 hours to cause polycondensation. In such a manner, awater soluble polymer having the structural units (a), the structuralunits (b), or the structural unit (a) and the structural unit (b) linkedvia the linking group represented by the formula (4) can be prepared.

Examples of the zinc plating bath using the zinc plating bath additiveof the present invention include an alkaline non-cyanide zinc platingbath which will be described below (which bath will hereinafter becalled “the alkaline non-cyanide zinc plating bath of the presentinvention”).

2. Alkaline Non-Cyanide Zinc Plating Bath:

The alkaline non-cyanide zinc plating bath of the present inventioncontains the zinc plating bath additive containing a water solublecopolymer having the structural units (a) and the structural units (b),zinc ions, and hydroxide ions. The term “water soluble copolymer” asused hereinafter means a water soluble copolymer having the structuralunits (a) and the structural units (b) unless otherwise specificallyreferred to. The term “zinc plating bath additive” as used hereinaftermeans a zinc plating bath additive (the above-described zinc platingadditive of the present invention) containing a water soluble copolymerhaving the structural units (a) and the structural units (b) unlessotherwise specifically referred to.

Electroplating using the alkaline non-cyanide zinc plating bath of thepresent invention enables the rapid formation of a zinc coating. Theresulting zinc coating has small variations in the thickness dependingon the position on the surface of the object. Even when the intensity ofthe current density on the surface of the object varies uponelectroplating, a zinc coating with small variations in thickness can beformed.

Further, in the alkaline non-cyanide zinc plating bath of the presentinvention, the higher the structural units (a)•(b) percentage in thewater soluble copolymer, the greater the deposition amount of zinc perhour. When more rapid formation of a zinc coating is required upon usingthe alkaline non-cyanide zinc plating bath of the present invention, itis only necessary to increase the structural units (a)•(b) percentage inthe water soluble copolymer. This means that the formation speed of azinc coating can be changed easily only by preparing a bath again whilereplacing only the water soluble copolymer with another one. Inaddition, in the alkaline non-cyanide zinc plating bath of the presentinvention, a formation speed of a zinc coating can be adjusted withoutchanging the zinc ion concentration of the bath or the temperature ofthe bath. This results in resolution of the problem such as excessivethickening of a zinc coating at limited positions on the surface of theobject which will otherwise occur when the zinc concentration in thebath or the bath temperature is increased. This means that the alkalinenon-cyanide zinc plating bath of the present invention can solve theproblem of the conventional process which has occurred at the time ofadjusting the formation speed of a zinc coating.

In the alkaline non-cyanide zinc plating bath of the present invention,the deposition amount of zinc per hour at a high current densityposition decreases with a decrease in the structural units (a)•(b)percentage of the water soluble copolymer. As a result, the depositionamount of zinc at a high current density position approaches thedeposition amount of zinc at a low current density position. It istherefore possible to make the deposition amount of zinc at a highcurrent density position and the deposition amount of zinc at a lowcurrent density position substantially equal to each other and therebyform a zinc coating with more uniform thickness by adjusting thestructural units (a)•(b) percentage of the water soluble copolymer.

The alkaline non-cyanide zinc plating bath of the present inventioncontains the water soluble copolymer having the structural units (a) andthe structural units (b) in an amount of preferably from 0.1 to 50 g/L.When from 0.1 to 50 g/L of the water soluble copolymer is contained inthe alkaline non-cyanide zinc plating bath of the present invention, anincrease or decrease of the structural units (a)•(b) percentage asdescribed above is likely to be reflected in an increase or decrease ofthe deposition amount of zinc per hour.

The alkaline non-cyanide zinc plating bath of the present inventioncontains preferably N-benzylpyrimidium-3-carbonate from the standpointof making the zinc coating brighter (enhancing the so-called brightness)or from the standpoint of improving the adherence of zinc deposited at alow current density position to the object (enhancing a throwing power).

From the standpoint of enhancing brightness or enhancing throwing power,the alkaline non-cyanide zinc plating bath of the present inventioncontains preferably a bisulfite adduct obtained by adding a bisulfite tothe aldehyde group of an aromatic aldehyde compound represented by thefollowing formula (5):

(in the above formula (5), R¹, R², and R³ each represents —H, —OH, or—OCH₃ and R¹ to R³ may be the same or different).

As the aromatic aldehyde compound, at least one compound selected fromthe group consisting of 4-methoxybenzaldehyde,4-hydroxy-3-methoxybenzaldehyde, 3,4-dimethoxybenzaldehyde,3,4-methylenedioxybenzaldehyde, 2-hydroxybenzaldehyde, and4-hydroxybenzaldehyde can be used.

The alkaline non-cyanide zinc plating bath of the present inventionpreferably contains the bisulfite adduct in an amount of from 0.005 to1.0 g/L.

The alkaline non-cyanide zinc plating bath of the present inventionpreferably contains zinc ions in an amount of from 5 to 25 g/L.

The alkaline non-cyanide zinc plating bath of the present invention canalso be used for a zincate zinc plating bath containing zincate zinc([Zn(OH)₄]²⁻). For the preparation of zincate zinc, a method ordinarilyemployed by those skilled in the plating technology can be adopted asneeded. For example, in order to obtain a plating bath having zincatezinc dissolved therein, zincate zinc may be prepared in a similar mannerto the conventionally known one by dissolving zinc oxide (ZnO) in anaqueous alkaline (NaOH) solution such as aqueous solution of sodiumhydroxide (NaOH).

The alkaline non-cyanide zinc plating bath of the present inventionpreferably contains at least one compound selected from the groupconsisting of silicon compounds, sodium ethylenediaminetetraacetate, andtrisodium hydroxyethylenediaminetriacetate for the purpose of renderingharmful metal ions, if any, harmless or for the purpose of softeningwater.

The alkaline non-cyanide zinc plating bath of the present inventionpreferably contains a heterocyclic compound represented by thebelow-described formula (6) and/or a derivative of the heterocycliccompound from the standpoint of increasing the deposition amount of zincper hour. In particular, inclusion of the heterocyclic compound or thederivative of the heterocyclic compound increases the deposition amountof zinc per hour at a low current density position and thereby decreasesa difference in the deposition amount of zinc per hour between the highcurrent density position and the low current density position. Thismeans that a zinc coating with a uniform thickness can be formed morerapidly by incorporating the heterocyclic compound or its derivativetherein.

(in the formula (6), p stands for an integer from 1 to 3 and R⁴ and R⁵may be the same or different and each represents —H, —CH₂COOH,—CH₂CH₂OH, or —CH₂CH(OH)CH₂Cl).

EXAMPLES

The present invention will next be described more specifically based onExamples. It should however be noted that the present invention is notlimited to or by these examples.

(1) Zinc Plating Bath Additive

(Additive A)

A 300-ml three-necked flask was charged with 19.3 g (0.084 mol) ofN,N′-bis[3-(dimethylamino)propyl]urea, 12.2 g (0.084 mol) of[3-(dimethylamino)propyl]urea, 3.3 g (0.084 mol) of sodium hydroxide,and 62.4 g of water, and then 15.4 g (0.17 mol) of epichlorohydrin wasadded in small portions at 60° C. or less under stirring. The reactionmixture was then heated under reflux for 2 hours to cause apolycondensation reaction. The polycondensate thus obtained was used asAdditive A.

(Additive B)

A 300-ml three-necked flask was charged with 25.7 g (0.112 mol) ofN,N′-bis[3-(dimethylamino)propyl]urea, 8.1 g (0.056 mol) of[3-(dimethylamino)propyl]urea, 2.2 g (0.056 mol) of sodium hydroxide,and 62.4 g of water, and then 15.4 g (0.17 mol) of epichlorohydrin wasadded in small portions at 60° C. or less under stirring. The reactionmixture was then heated under reflux for 2 hours to cause apolycondensation reaction. The polycondensate thus obtained was used asAdditive B.

(Additive C)

A 300-ml three-necked flask was charged with 30.9 g (0.134 mol) ofN,N′-bis[3-(dimethylamino)propyl]urea, 4.9 g (0.034 mol) of[3-(dimethylamino)propyl]urea, 1.3 g (0.034 mol) of sodium hydroxide,and 62.4 g of water, and then 15.4 g (0.17 mol) of epichlorohydrin wasadded in small portions at 60° C. or less under stirring. The reactionmixture was then heated under reflux for 2 hours to cause apolycondensation reaction. The polycondensate thus obtained was used asAdditive C.

(Additive D)

A 300-ml three-necked flask was charged with 38.6 g (0.168 mol) ofN,N′-bis[3-(dimethylamino)propyl]urea and 62.4 g of water, and then 15.4g (0.17 mol) of epichlorohydrin was added in small portions at 60° C. orless under stirring. The reaction mixture was then heated under refluxfor 2 hours to cause a polycondensation reaction. The polycondensatethus obtained was used as Additive D.

(Additive E)

A polycondensate (trade name Product, Product No. 3-138N) available bycharging dimethylaminopropylamine and epichlorohydrin at a molar ratioof 1:1 to cause a polycondensation reaction was obtained from URSA andused as Additive E.

The molecular weight of Additives A to E was measured based on acalibration curve, which was a third-order approximation curve withpolyethylene oxide (PEO) as a standard sample, by using as ahigh-performance GPC apparatus HLC-8320GPC and Eco SEC (product ofTOSOH) (GPC measurement conditions will be described later). As aresult, it has been found that Additive A has Mw of 3900, Additive B hasMw of 4600, Additive C has Mw of 5700, Additive D has Mw of 7200, andAdditive E has Mw of 16000. GPC is a method of separating a targetsubstance on the basis of molecular size. The GPC apparatus is anapparatus for separating a substance by using a chromatograph equippedwith a column capable of sorting it based on the molecular size. GPC isexcellent in separating/analyzing particularly a high molecularsubstance. Aqueous GPC is a kind of size exclusion chromatography and isabbreviated as SEC.

<Conditions common to GPC measurement>

Aqueous GPC apparatus: HLC-8320GPC EcoSEC (product of TOSOH TechnoSystem)

Column: TSKgel G6000PWXL-CP TSKgel G3000PWXL-CP (7.8 mm I.D.×30 cm)

Detector: differential refractometer (RI detector)

Eluent: 0.1M NaNO₃ aqueous solution

Column temperature: 40° C.

The mass ratio of the structural units (a) and (b) of the polymercontained in Additives A to D was determined using ¹³C-NMR (DMSO-d6, 400MHz). The mass of the structural units (a) was calculated based on theintensity ratio of their carbonyl carbon peak at about 162 ppm and thatof the structural units (b) was calculated based on the intensity ratioof their carbonyl carbon peak at about 159 ppm.

A percentage of the structural units (a)_(m)(b) [mass of the structuralunits (a)/(mass of the structural units (a)+mass of the structural units(b)×100) of the polymer contained in Additives A to D are shown in Table1.

TABLE 1 Percentage (%) of structural units (a) · (b) Additive A 50Additive B 33 Additive C 20 Additive D  0

(2) Preparation of Alkaline Non-Cyanide Zinc Plating Bath Example 1

An alkaline non-cyanide zinc plating bath containing 12 g/L of zincions, 120 g/L of sodium hydroxide, 2.50 g/L of Additive A, and 0.05 g/Lof a 48% aqueous solution of benzyl pyridinium carboxylate [“LugalvanBPC-48”, product of BASF, which will hereinafter be called “brightener(I)”] was prepared (Table 2).

Example 2

An alkaline non-cyanide zinc plating bath containing 12 g/L of zincions, 120 g/L of sodium hydroxide, 2.50 g/L of Additive B, and 0.05 g/Lof the brightener (I) was prepared (Table 2).

Example 3

An alkaline non-cyanide zinc plating bath containing 12 g/L of zincions, 120 g/L of sodium hydroxide, 2.50 g/L of Additive C, and 0.05 g/Lof the brightener (I) was prepared (Table 2).

Example 4

An alkaline non-cyanide zinc plating bath containing 12 g/L of zincions, 120 g/L of sodium hydroxide, 2.50 g/L of Additive B, and as abrightener, 0.03 g/L of an aqueous solution of a Na bisulfite adduct ofanisaldehyde (a content of the Na bisulfite adduct of anisaldehyde was40 g/L in terms of anisaldehyde) [which will hereinafter be called“brightener (II)”] was prepared (Table 2).

Comparative Example 1

An alkaline non-cyanide zinc plating bath containing 12 g/L of zincions, 120 g/L of sodium hydroxide, 2.50 g/L of Additive D, and 0.05 g/Lof the brightener (I) was prepared (Table 2).

Comparative Example 2

An alkaline non-cyanide zinc plating bath containing 12 g/L of zincions, 120 g/L of sodium hydroxide, 1.60 g/L of Additive E, and as abrightener, 0.05 g/L of the brightener (I) was prepared (Table 2).

Comparative Example 3

An alkaline non-cyanide zinc plating bath containing 12 g/L of zinc ionsand 120 g/L of sodium hydroxide was prepared (Table 2).

TABLE 2 Evaluation of uniform electrodeposition Burnt Average HCD LCDdeposit coating coating coating Composition of plating bath at high Bendtest at thick- thick- thick- Uniform Zn NaOH Additive Brightener currenthigh current ness ness ness electro- (g/L) (g/L) Kind (g/L) Kind (g/L)Brightness area area (μm) (μm) (μm) deposition Example 1 12 120 AdditiveA 2.50 Brightener 0.05 Entire None No peeling 4.91 6.28 3.45 1.82 (I)brightness Example 2 12 120 Additive B 2.50 Brightener 0.05 Entire NoneNo peeling 3.91 4.48 3.22 1.39 (I) brightness Example 3 12 120 AdditiveC 2.50 Brightener 0.05 Entire None No peeling 3.06 3.25 2.78 1.17 (I)brightness Example 4 12 120 Additive B 2.50 Brightener 0.03 Entire NoneNo peeling 4.23 4.97 3.19 1.56 (II) brightness Comp. Ex. 1 12 120Additive D 2.50 Brightener 0.05 Entire None No peeling 1.00 1.05 1.031.02 (I) brightness Comp. Ex. 2 12 120 Additive E 1.60 Brightener 0.05Semi- Observed Peeling 4.92 8.20 2.04 4.02 (I) brightness observed Comp.Ex. 3 12 120 Free — Free — Coarse Observed Peeling 7.44 8.67 2.70 3.21crystals observed

(3) Hull Cell Plating Test

In a plating tank (a 267-ml hull cell tank, product of YAMAMOTO MS Co.,Ltd., Product No: B-50 plain type tank) containing any one of alkalinenon-cyanide zinc plating baths obtained in Examples 1 to 4 andComparative Examples 1 to 3, an iron sheet (45 mm long×45 mm wide×1 mmthick) was placed as an anode sheet and a cold rolled steel sheet (SPCC)(65 mm long×100 mm wide×1.5 mm thick) was placed as a cathode sheet.Plating was performed under plating conditions of 2.0 A and 10 minutesat a zinc plating bath temperature of 25° C.

(4) Evaluation of Brightness and Appearance

After the above-described hull cell plating test, the brightness of azinc coating formed on the surface of a cathode sheet was visuallyjudged. The results are shown in Table 2. The term “entire brightness”as used in Table 2 means that a zinc coating has substantially a uniformmirror surface. The term “semi-brightness” means that a zinc coating hasdull brightness. The term “coarse crystals” means crystals which aredifferent in size and are therefore coarse.

(5) Bend Test at High Current Area

FIG. 1 is a schematic view showing the front surface (surface subjectedto zinc plating which is so-called “plating surface”) of a cathode sheetafter the hull cell plating test. Referring to FIG. 1, the cathode sheetwas bent so as to form a fold line extending from the end of a highcurrent area to the end of a low current area at a positioncorresponding to just the center depth when the cathode sheet was placedin a plating tank. Described specifically, the cathode sheet was bent180 degrees to bring the halved back surfaces (on the side opposite tothe plating surface) of the cathode sheet in contact with each other andthen the cathode sheet was unfolded until the bent plating surfacebecame flat. Under such a state, whether peeling of the zinc coatingoccurred at the fold line in the vicinity of the end of the high currentarea (within a range of 20 mm from the end of the high current area) onthe plating surface was judged. The results are shown in Table 2.

(6) Measurement of Thickness of Zinc Coating

The thickness of the zinc coating was measured using a fluorescent X-raycoating thickness gauge (SFT-9200, product of SII). Referring to FIG. 1,the coating thickness was measured at nine positions in total, that is,10 mm, 20 mm, 30 mm, 40 mm, 50 mm, 60 mm, 70 mm, 80 mm, and 90 mm towardthe end of the low current area from the end of the high current area ata position corresponding to just the center depth (similar to theposition where a fold line was made in the above-described bend test athigh current area) when a cathode sheet was placed in a plating tank.The results of Examples 1 to 4, Comparative Example 1, and ComparativeExample 2 are shown in the graph of FIG. 2. The results of ComparativeExample 3 are shown in the graph of FIG. 3.

(7) Evaluation of Uniform Electrodeposition

Uniform electrodeposition was evaluated based on the calculated resultsof a ratio (HCD/LCD) of the coating thickness (HCD) at a position 20 mmfrom the end of the high current area to the coating thickness (LCD) ata position 80 mm from the end of the high current area in themeasurement of the thickness of the zinc coating. The results are shownin Table 2.

The results have revealed that in Examples 1 to 4, uniformelectrodeposition is high and a thick zinc coating can be formed. InComparative Example 1, on the other hand, a uniformly thin coatingextends from a high current density position to a low current densityposition and thus, a zinc coating with uniform thickness is formed. InComparative Example 2, a zinc coating becomes excessively thick at ahigh current density position, while the zinc coating becomes thin at alow current density position. This means that when Additive E wascontained as in Comparative Example 2, the resulting zinc coatingbecomes excessively thick at some positions on the surface of the objectunder the following conditions: zinc ion concentration of 12 g/L andtemperature of zinc plating bath of 25° C.

In addition, it has been elucidated from the results of Examples 1 to 3that the formation speed of a zinc coating can be increased or decreasedby increasing or decreasing the structural units (a)•(b) percentage ofthe water soluble copolymer while keeping the concentration of the watersoluble copolymer constant.

It has been elucidated that using the zinc plating bath of Example 3enables the formation of a thick zinc coating with uniform thickness. Onthe other hand, it has been elucidated that a zinc coating obtainedusing the zinc plating bath of Comparative Example 1 has a uniformthickness but is thin (the thickness of the zinc coating of ComparativeExample 1 was about one third of the thickness of the zinc coating ofExample 3).

Comparative Example 3 does not contain any of Additives A to E. As shownin FIG. 3, it has been confirmed that when a zinc plating bath does notcontain an additive having an action of suppressing deposition of zinc,the zinc coating has variations in thickness, depending on the positionon the surface of the object. In Comparative Example 3, the coatingbecomes excessively thick at some positions irrespective of theintensity of the current density (a zinc coating with an uneven surfacewas formed).

INDUSTRIAL APPLICABILITY

The present invention is useful as a zinc plating bath additive to beadded to a zinc plating bath used in electroplating and an alkalinenon-cyanide zinc plating bath.

1-12. (canceled)
 13. A zinc plating bath additive comprising a watersoluble copolymer having structural units (a) each represented by thefollowing formula (1) and structural units (b) each represented by thefollowing formula (2):


14. The zinc plating bath additive according to claim 13, wherein thestructural units (a) and the structural units (b) are contained in thewater soluble copolymer so that a percentage of the mass of thestructural units (a) relative to the sum of the mass of the structuralunits (a) and the mass of the structural units (b) is from 10 to 60%.15. The zinc plating bath additive according to claim 13, wherein in thewater soluble copolymer, the structural units (a), the structural units(b), or the structural unit (a) and the structural unit (b) have beenlinked to each other via a linking group represented by the followingformula (3) and/or the following formula (4):

(in the formula (3), a stands for an integer from 1 to 5, b stands foran integer from 1 to 5, d stands for an integer from 1 to 5, e standsfor an integer from 1 to 5, and m stands for an integer from 0 to 5,while in the formula (4), f stands for an integer from 1 to 5, g standsfor an integer from 1 to 5, h stands for an integer from 1 to 5, istands for an integer from 1 to 5, and n stands for an integer from 0 to5).
 16. The zinc plating bath additive according to claim 14, wherein inthe water soluble copolymer, the structural units (a), the structuralunits (b), or the structural unit (a) and the structural unit (b) havebeen linked to each other via a linking group represented by thefollowing formula (3) and/or the following formula (4):

(in the formula (3), a stands for an integer from 1 to 5, b stands foran integer from 1 to 5, d stands for an integer from 1 to 5, e standsfor an integer from 1 to 5, and m stands for an integer from 0 to 5,while in the formula (4), f stands for an integer from 1 to 5, g standsfor an integer from 1 to 5, h stands for an integer from 1 to 5, istands for an integer from 1 to 5, and n stands for an integer from 0 to5).
 17. An alkaline non-cyanide zinc plating bath comprising the zincplating bath additive as claimed in claim 13, zinc ions, and hydroxideions.
 18. An alkaline non-cyanide zinc plating bath comprising the zincplating bath additive as claimed in claim 14, zinc ions, and hydroxideions.
 19. An alkaline non-cyanide zinc plating bath comprising the zincplating bath additive as claimed in claim 15, zinc ions, and hydroxideions.
 20. An alkaline non-cyanide zinc plating bath comprising the zincplating bath additive as claimed in claim 16, zinc ions, and hydroxideions.
 21. The alkaline non-cyanide zinc plating bath according to claim17, wherein the water soluble copolymer is contained in an amount offrom 0.1 to 50 g/L.
 22. The alkaline non-cyanide zinc plating bathaccording to claim 17, comprising N-benzylpyrimidium-3-carbonate. 23.The alkaline non-cyanide zinc plating bath according to claim 17,comprising a bisulfite adduct obtained by adding a bisulfite to thealdehyde group of an aromatic aldehyde compound represented by thefollowing formula (5):

(in the formula (5), R¹, R², R³ each represents —H, —OH, or —OCH₃ and R¹to R³ may be the same or different).
 24. The alkaline non-cyanide zincplating bath according to claim 23, wherein the aromatic aldehydecompound is at least one compound selected from the group consisting of4-methoxybenzaldehyde, 4-hydroxy-3-methoxybenzaldehyde,3,4-dimethoxybenzaldehyde, 3,4-methylenedioxybenzaldehyde,2-hydroxybenzaldehyde, and 4-hydroxybenzaldehyde.
 25. The alkalinenon-cyanide zinc plating bath according to claim 23, wherein thebisulfite adduct is contained in an amount of from 0.005 to 1.0 g/L. 26.The alkaline non-cyanide zinc plating bath according to claim 17,wherein the zinc ions are contained in an amount of from 5 to 25 g/L.27. The alkaline non-cyanide zinc plating bath according to claim 17,comprising at least one compound selected from the group consisting ofsilicon compounds, sodium ethylenediaminetetraacetate, and trisodiumhydroxyethylenediaminetriacetate.
 28. The alkaline non-cyanide zincplating bath according to claim 17, comprising a heterocyclic compoundrepresented by the following formula (6) and/or a derivative of theheterocyclic compound:

(in the formula (6), p stands for an integer from 1 to 3 and R⁴ and R⁵may be the same or different and each represents —H, —CH₂COOH,—CH₂CH₂OH, or —CH₂CH(OH)CH₂Cl).